The present invention relates to a method of fabricating contact electrodes in semiconductor devices such as integrated circuits, transistors, and diodes. The method of this invention is particularly well applicable to the fabrication of contact electrodes in power-handling semiconductor devices in which the insulating films to be penetrated are generally thicker than in other semiconductor devices.
As semiconductor devices have grown finer in design in recent years, so have become less in size the contact holes that must be formed in and through their insulating films for creation of electrodes. In addition to that, at least as far as power-handling semiconductor devices are concerned, the insulating films in which are to be formed the contact holes remain relatively thick in order to withstand high voltages. The so-called aspect ratio of the contact holes, the ratio of hole depth to diameter, have become all the more higher in this type of semiconductor devices, making it proportionately more difficult to fill metal, such as aluminum, into these holes. An incomplete packing of the contact holes with metal is of course undesirable from the standpoint of reduction of contact resistance to a minimum.
A conventional remedy to this problem is what is known as the contact reflow process, which starts with the creation of a hole in a film of electrically insulating material on a semiconductor substrate, as by the more conventional method of photolithography or etching. The insulating film may be of either boron-phosphor-silicate glass (BSPG) or phosphor-silicate glass (PSG). Anisotropic etching is considered desirable for creation of a hole that extends approximately perpendicular to the substrate surface.
The next step is the heating of the insulating film to such a temperature that it undergoes deformation, or reflow, with the consequent flaring of the hole as it extends away from the substrate surface, or tapering thereof as it extends toward the substrate surface. Even though the insulating film may be relatively thick, and the original hole correspondingly high in aspect ratio, the tapering contact hole thus formed is bound to accept metal far more easily and more thoroughly than if it were constant in diameter, as the metal is introduced as by vacuum deposition. The result is an improvement in the so-called step coverage of the electrode.
The contact reflow process has its own shortcoming, however. Upon heating, as above, of the insulating film to a reflowing temperature following the creation of a hole therein, there occurs the so-called outward diffusion of such substances as phosphor and boron contained therein. These substances, especially boron, find their way onto the substrate surface exposed through the contact hole thereby preventing favorable electrical contact of the electrode with the substrate.
An obvious solution to this weakness of the contact reflow process might seem to create, as by thermal oxidation, a silicon oxide film on the substrate surface forming the bottom of the contact hole, preparatory to the heat treatment of the insulating film. This solution would be impractical because the noted outward diffusion of boron and the like would occur during creation of the silicon oxide film, to such an extent that the substrate surface would not be satisfactorily kept from contamination by the impurities.